1. Field of the Invention
The present invention relates to satellite monitoring, and particularly, but not exclusively, to a method, apparatus, system and computer program for terrestrial monitoring of the transmission performance of a multi-beam satellite.
2. Background Art
Operators of satellite systems need to monitor various transmission properties of their satellites, such as the centre frequencies of frequency channels, carrier to noise ratios (C/No), link quality and Effective Isotropic Radiated Power (EIRP). Some or all of these properties may be measured directly or indirectly by receiving user terminals and reported back to the system, for example to assist in power control, Doppler correction or variable data rate techniques. However, the availability and geographical spread of user terminals is outside the control of the satellite operator, which cannot therefore rely on user terminals for comprehensive monitoring of the transmission properties of a satellite.
Hence, there is a need for permanently active satellite monitoring stations located in representative geographical locations. As an example, a satellite monitoring system currently used for the applicant's Inmarsat-3™ satellites will now be described with reference to FIG. 1.
The Inmarsat™ mobile satellite communications system includes a plurality of geostationary Inmarsat-3™ satellites 2, one of which is shown in FIG. 1. The satellite 2 generates a global beam 6 and five spot beams 8a-e which fall within the global beam 6, the beam patterns being substantially coterminous for transmission and reception. The spot beams 8a-e are used predominantly for communications traffic, while the global beam 6 is used predominantly for call set-up and communications traffic outside the coverage of the spot beams 6.
For each satellite 2, a plurality of land earth stations (LES) 4a-b act as satellite base stations and gateways to terrestrial networks. Each LES 4 communicates at C-band over a bidirectional feeder link 10 via the satellite 2, which maps frequency channels within the feeder link 10 to corresponding beams and L-band channels within the beams, according to a variable channel mapping configured on the satellite 2 under the control of a telemetry, tracking and control (TT&C) station (not shown).
To monitor the spot beams 8, a remote monitoring station (RMS) 12 needs to be located in each spot beam 8. The RMS 12 receives a current frequency plan, monitors L-band channels within the relevant spot beam or beams 8, and records channel measurements from which the required transmission properties of the satellite 2 can be derived. Each RMS must be kept operational as near continuously as possible, and must be calibrated so that the measurement results are reliable; therefore, it is convenient to collocate RMSs 12a, 12b with LESs 4a, 4b so that existing maintenance facilities can be used.
Moreover, the RMSs 12 must transmit monitoring data so that it can be processed by a central server. The data may be transmitted over the satellite network, or over a wireline network such as an ISDN. Therefore, collocated RMSs 12a, 12b have the advantage of being able to use existing communications facilities at the LESs 4a, 4b to transmit this data.
If an LES 4b is located where two spot beams 8c, 8d overlap, the collocated RMS 12b is able to monitor both spot beams 8c, 8d, thus reducing the number of RMSs 12 required.
For those spot beams 8 which do not contain an LES 4, a transportable monitoring station (TMS) 12c, 12d may be provided. The TMSs 12c, 12d are conveniently located where suitable maintenance and/or terrestrial communication facilities are available. However, it is more difficult to provide the necessary maintenance and communications facilities to the TMSs 12c, 12d than to collocated RMSs 12a, 12b. 
Whilst the above system is acceptable for monitoring satellites with a small number of spot beams, problems arise in adapting the system for satellites where the number of spot beams is very much greater. For example, the proposed Inmarsat-4™ satellites will generate up to 19 regional beams and 256 spot beams, most of which will not cover an existing LES 4. A very large number of TMSs 12, with a diverse geographical distribution, would be needed to ensure that every regional and spot beam contains at least one monitoring station 12. It would be extremely difficult to maintain such a large number of TMSs, particularly as some spot beams would cover only marine or mountainous areas.
Moreover, the Inmarsat-4™ satellites will have reconfigurable beam patterns, so that a distribution of monitoring stations 12 adequate to monitor one beam pattern configuration may not be adequate to monitor another.
The problems described above are not unique to Inmarsat™ satellite communications systems. As the demand for high-bandwidth satellite communications increases, the number of spot beams required also increases, to provide the necessary gain and frequency re-use for high bandwidth services. The problems are not unique to geostationary satellites, and may be more acute for non-geostationary satellites which generate a moving beam pattern. The problems are not unique to repeater satellites, and may be more acute with switching satellites, which may have fewer terrestrial gateways which can be used for satellite monitoring.
The document U.S. Pat. No. 5,710,971 discloses a satellite monitoring system, for call interception rather than monitoring transmission performance.